Track Pitch Variation

MagneTrackTM measures MFM images.

To put more data on a disk, you must decrease track pitch.
But when the tracks are too close together, cross-talk causes data errors.
In an ideal disk drive, the servo marks are placed at equal radial intervals and
the tracking servo responds perfectly to those marks; the track spacing can be
as small as allowed by the magnetic performance of the media and the head.
In a real drive, track pitch variation requires an increase (added tolerance) in
the target track pitch. Now you can analyze Magnetic Force Microscope (MFM)
images to obtain this information directly, with nanometer-scale accuracy,
without relying on an indirect calibration.

MFM image of data bits on a hard disk The Magnetic Force Microscope senses
the magnetic field just above the disk surface. 20 micron scan.

Track Pitch results from 4 MFM images in the data region of a hard disk.
The standard deviation of track pitch is a tool used by disk design engineers to
set track pitch target values in each generation of disk drive.